Vibration damped sandwich systems

ABSTRACT

A VIBRATION DAMPED SANDWICH SYSTEM COMPRISING TWO HARD PLATES AND INTERPOSED BETWEEN THE PLATES A VIBRATION DAMPING INTERLAYER COMPRISING GRAFT POLYMERS OF STYRENE OR OPTIONALLY STYRENE WITH SMALL AMOUNTS OF A COPOLYMERIZABLE CARBOXYLIC ACID, ESPECIALLY ACRYLIC ACID AND/OR METHACRYLIC ACID, ON A COPOLYMER OF 30 TO 40% BY WEIGHT OF VINYL ACETRATE, 30-40% BY WEIGHT OF 2-ETHYLHEXYL ACRYLATE, 30 TO 10% BY WEIGHT OF DIBUTYL MALEATE AND 5 TO 15% BY WEIGHT OF CROTONIC ACID.

Feb. 9,A 1971 Filed Nov. 18, 196e H. oBERsT ET AL 3,562,091

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llllllllllll'llllllllll INVENTORS HERMANN @eener JQACHIM 551er @Umm-tnoms-z ALFRED SCHOMMEP? United States Patent O U.S. Cl. 161-166 4 ClaimsABSTRACT OF THE DISCLOSURE A vibration damped sandwich system comprisingtwo hard plates and interposed between the plates a vibration dampinginterlayer comprising graft polymers of styrene or optionally styrenewith small amounts of a copolymerizable carboxylic acid, especiallyacrylic acid and/or methacrylic acid, on a copolymer of 30 to 40% byweight of vinyl acetate, 30-40% by weight of Z-ethylhexyl acrylate, 30to 10% by weight of dibutyl maleate and 5 to by weight of crotonic acid.

The present invention provides vibration damped sandwich systems havinginterlayers made of graft polymers of styrene or optionally styrene withsmall amounts of a copolymerizable carboxylic acid, especially acrylicacid and/or methacrylic acid, on vinyl acetate/Z-ethylhexylacrylate/dibutyl maleate/crotonic acid copolymers.

It is known from South African specification No. 5,269 that highlyvaluable vibration damping materials of a broad temperature bandsuitable for damping bending vibrations of metal sheet constructions canbe prepared by copolymerizing monomers whose homopolymers differ intheir second order transition temperature by at least C. The abovespecification also reports that as vibration damping materials having abroad temperature band there can be used, among others, predominantlyamorphous copolymers of esters of alcohols with 4 to 12 carbon atomswith acrylic and maleic acid and vinyl esters of fatty acids with 2 to 3carbon atoms, for example vinyl acetate/2-ethylhexyl acrylate and/ordibutyl maleate copolymers.

It has now been found that graft polymers of styrene or optionallystyrene with small amounts of a copolymerizable carboxylic acid,especially acrylic acid and/or methacrylic acid, on copolymers of vinylacetate, suitable esters of unsaturated polymerizable carboxylic acidsand an unsaturated copolymerizable acid, preferably crotonic acid haveoutstanding damping properties, a large temperature range of damping andare thus especially suitable for the vibration damping of sandwichsystems of hard plates, in particular metal sheets. Suitable esters areespecially those of acrylic acid and maleic acid with alcohol componentshaving 3 to 12 carbon atoms, preferably 2-ethylhexyl acrylate anddibutyl maleate. With these polymers, the vibration damping effect ofwhich critically depends on the weight proportion of the monomers, verybroad damping curves can be obtained having very high maximum dampingvalues. From the economical point of view they have the advantage ofbeing rather cheap.

The graft polymers of styrene or optionally styrene with acopolymerizable carboxylic acid (acrylic acid and/ or methacrylic acid)on copolymers of 30 to 40% by weight of vinyl acetate, 30 to 40% byweight of 2-ethylhexyl acrylate, 30 to 10% by weight of dibutyl maleateand about 10% by weight of crotonic acid are produced Ice by freeradical initiated polymerization at a temperature in the range of fromabout 60 to 180 C. by dissolving the copolymer in monomeric styrene oroptionally styrene in admixture with a copolymerizable carboxylic acid(acrylic acid and/or methacrylic acid). Especially good results areobtained with graft polymers of 40 to 80% by weight of styrene orstyrene together with a copolymerizable carboxylic acid (acrylic acidand/or methacrylic acid) on to 60 to 20% by weight of the copolymersspecified above, for example a graft polymer of 50% by weight of amixture of by weight of styrene and 10% by weight of acrylic acid on to50% by weight of the specified copolymer. As initiator in thepolymerization there may be used tertiary butylhydroperoxide in theusual concentration.

The present invention provides sandwich systems of hard plates, inparticular metal sheets, having vibration damping self-adherentinterlayers of graft polymers of styrene or optionally styrene togetherwith a copolymerizable carboxylic acid (acrylic or methacrylic acid) onvinyl acetate/Z-ethylhexyl acrylate/dibutyl maleate/crotonic acidcopolymers, for which interlayers there are used graft polymers of 40 to80% by weight of styrene or styrene together with a copolymerizablecarboxylic acid (acrylic acid and/ or methacrylic acid) on to 60 to 20%by weight of copolymer consisting of 30 to 40% by weight of vinylacetate, 30 to 40% by weight of Z-ethylhexyl acrylate, 30 to 10% byweight of dibutyl maleate and 5 to 10% by weight of crotonic acid.

FIGS. 1a to 1c of the accompanying drawings are plots againsttemperature of the loss factor dcomb to illustrate the superior eciencyof the novel systems. The curves in FIGS. 1a and 1b show the loss factordcomb of two metal sheet arrangements of the invention as a function oftemperature. For comparison, one of the most effective vibration dampingmaterials known for metal sheet arrangements was used, namely acopolymer of 63% by weight of vinyl acetate and 37% by weight of dibutylmaleate containing as plasticizer 15% by weight of 2- ethylhexylphthalate and 15% by Weight of tricresyl phosphate, calculated on themixture (curve FIG. 1c). The copolymer of curve 1c was a thermoplasticadhesive especially suitable for producing vibration damped metal sheetsandwich systems comprising two outer metal sheets and a self-adherentthermoplast as damping interlayer. Systems of this type provide adamping effect which is extremely high in its maximum and cannot beexceeded for physical reasons (cf. H. Oberst and A. Schommer,Kunststoffe, volume 55, page 634 (1965), especially FIG. 9). In asymmetrical arrangement comprising two metal sheets, each having athickness of 0.5 millimeter, and an interlayer 0.3 millimeter thick, theloss factor dcomb of the combined system, measured in the bending wavemethod (cf. for example H. Oberst, L. Bohn and F. Linhardt, Kunststoffe,volume 51, page 495 (1961)), almost reaches the value dcomb of 1. Theknown metal sheet damping by one-side damping coatings which are appliedby spraying, trowelling or bonding in the form of layers of so-calledvibration damping materials show loss factors generally of less thandc0mb=0-2 with technically reasonable thicknesses or ratios of coatingmass to plate mass of the combined system. With metal sheet sandwichsystems which gain growing importance in recent times, it is possible toobtain damping Values that are increased by a multiple, as shown by thepresent example, when the interlayer material has the appropriatecomposition and thickness.

The temperature band width of the damping of the metal sheet sandwichsystem does not only depend on the viscoelastic properties of theinterlayer and the steel sheets but also to a considerable extent on thegeometry of the arrangement, i.e. on the ratio of the layer thicknesses(cf.

loc. cit (1965), FIGS. 8 to 10). With metal sheet sandwich systems theband width is advantageously dened as the range of the temperatureinterval within which the value dcmb=0-05 is exceeded. The damping ofmetal sheets which are not damped by additional vibration damping meansin metal sheet constructions of various types corresponds to valuesricambi-0.01. The reference value dcomb of 0.05 thus means aconsiderable increase in the damping effect (by about 15 db (decibel))as cornpared to the nil damping dcomb=0.01.

In the curve shown in FIG. 1c the reference value dcomb of 0.05 isexceeded in the mainly interesting frequency range of from 100 to 1,000c.p.s. (Hz.) at temperatures ranging from about 0 to 50 C. Thetemperature band width thus corresponds to about 50 C. Sandwich systemsof this type are suitable for many technical fields of application. Bymodifying the content of plasticizer, it is possible to shift thetemperature band of a high damping effect to higher temperatures andthus to adapt the material to special technical uses, for example inmachine units operating at elevated temperatures. This example of ametal sheet sandwich system comprising a self-adherent interlayer havingoptimum properties of a vibration damping material with a broadtemperature band prepared by copolymerizing appropriate monomericcompounds has hitherto not been surpassed by other arrangements ofsimilar kind and may be taken as standard for judging the acousticefficiency of the system according tothe invention.

FIGS. 1a to 1c show the temperature curves of the loss factor dem, ofmetal sheet sandwich systems comprising steel sheets of a thickness of0.5 millimeter each and damping interlayers of a thickness of 0.7 and0.3 millimeter (FIG. 1b) for a frequency of 100 c.p.s. and 1,000 c.p.s.

The curves were measured with sandwich systems the interlayers of whichconsisted of:

(la) A graft polymer of 50% by weight of styrene on 50% by weight of acopolymer of 35% by weight of vinyl acetate, 35 by weight of2-ethylhexyl acrylate, 20% by weight of dibutyl maleate and 10% byweight of crotonic acid (according to the invention).

(1b) A graft polymer of 50% by weight of a mixture of 90% by weight ofstyrene and 10% by weight of acrylic acid on to 50% by weight of acopolymer of 35% by Weight of vinyl acetate, 35% by weight of2-ethylhexyl acrylate, 20% by weight of dibutylmaleate and 10% by weightof crotonic acid (according to the invention).

(1c) A copolymer of 63% by weight of vinyl acetate and 37% of dibutylmaleate containing as plasticizer 15% by weight of 2-ethylhexylphthalate and 15 by weight of tricresyl phosphate, calculated on themixture.

The arrangements 1a and 1b according to the invention, whose monomerproportions lie in the optimum range have very broad temperature bandwidths with relatively high maximum damping values that come near to thevalues of arrangement 1b. With the arrangement 1a the centre of dampingis at 35 to 55 C. with maximum damping values of about 0.5 to 0.8. Thetemperature band width is about 145 C. for 100 c.p.s. and about 110 C.for 1,000 c.p.s. The excellent vibration damping properties aremaintained at a temperature of about 5 C. to about 120 C. As comparedwith curve 1c the slow decrease of damping towards high and lowtemperatures is very advantageous. With the arrangement 1b the center ofdamping is also in the temperature range of about 35 to 55 C. withmaximum damping values daomb of 0.5 to about 0.8. The temperature bandwidth is about 140 C. (for 100 c.p.s.) and about 115 C. (for 1,000c.p.s.). As in the arrangement 1a, in this case, too, the slow decreaseof the damping toward high and low temperatures is especially favorable.The reference value dcomb of 0.05 is exceeded at a temperature in therange of from about 5 C. to about 130 C. As compared with the standardsystem 1c, arrangements 1a and 1b have appreciably broader temperatureband widths and better damping properties at temperatures above 30 C. sothat they may be used for many applications in machines and appliancesoperated at normal and elevated temperatures. Owing to their content of10% by weight of crotonic, acrylic and/ or methacrylic acid the graftpolymers used in arrangements 1a and 1b can be cross-linked by reactionwith a bifunctional or trifunctional compound (for example a compoundcontaining a plurality of epoxide, isocyanate or similar groups),whereby the softening range and therewith the range of high damping canbe noticeably shifted toward higher temperatures for specialapplications.

A special advantage of the vibration damping material of the presentinvention resides in the fact that it may be applied continuously duringthe mass production of the metal sheet sandwich systems.

For this purpose the vibration damping material can be used (1) in theform of the graft polymer, (2) in the form of a solution of thespecified copolymer in the styrene or mixture of styrene with acrylicand/or methacrylic acid to be grafted on which contains thepolymerization initiator and the graft polymer can be produced by athermal treatment of the sandwich system comprising the said solution asinterlayer at a temperature in the range of from about 60 to about 180C.

It is a thermoplastic adhesive which may be applied to the metal sheetsby trowelling, brushing or pouring at elevated temperature. The sandwichsystem may then be advantageously cooled under pressure between rollers.Except for degreasing the metal sheets do not require a preliminarytreatment and further adhesive. Owing to the content of the graftpolymer of crotonic or acrylic and/or methacrylic acid degreasing mayeven be dispensed with.

' The adhesion is very good.

The vibration damping material of the present invention has a goodresistance to flow. The metal sheet sandwich system may, within broadlimits, be processed as normal metal sheets, that is they may becreased, bent, shaped, welded and riveted. In this manner laminatedsystems are obtained having a damping height and temperature range ofdamping which make them well suitable for many applications at normaland elevated temperatures.

Minor amounts of llers, for example for improving the electricconductivity (improvement of resistance welding) may be incorporated inthe vibration damping materials. In order not to affect the dampingeffect adversely it is advantageous to use less than 1% by weight,preferably less than 0.5% by weight of filler, calculated on thepolymer. Suitable fillers are, for example, heavy spar, silicic acid,graphite and soot.

The metal sheet sandwich system suitably has a total thickness in therange of from 1 to 6 millimeters. The interlayers may have a thicknessof 0.1 to 1 millimeter, preferably 0.2 to 0.5 millimeter. A maximumdamping effect is obtained with symmetrical laminated systems. With anequal weight, asymmetrical laminated systems have, however, a higherstiffness in flexure and strength. Asymmetrical laminated systems are,therefore, preferred for those applications which require a highstrength with respect to the weight. The ratio of the thicknesses of thecuter plates or metal sheets is preferably within the range of from 1:1to 1:4.

FIG. 2 of the accompanying drawings shows sandwich systems withsymmetrical arrangement (a) and asymmetrical arrangement (b) in whichthe interlayer 2 is interposed between the two outer plates or metalsheets 1.

What is claimed is:

1. A vibration damped sandwich system comprising two hard plates andinterposed between the plates a vibration damping self-adherentinterlayer comprising a graft polymer of 40 to 80% by weight of styreneor a mixture of styrene with 0.1 to 10% by weight of acrylic and/ormethacrylic acid, calculated on the styrene, on 60 to 20% by weight of acopolymer of 30 to 40% by weight of vinyl acetate, 3040% by weight ofZ-ethylhexyl acrylate, 30 to 10% by Weight of dibutyl maleate and 5 to15% by weight of crotonic acid.

2. A vibration damped sandwich system as claimed in claim 1, `whereinthe hard plates are metal sheets.

3. A vibration damped sandwich as claimed in claim 1, wherein the graftpolymer of the interlayer contains up to 1% by weight of a filler,calculated on the graft polymer.

4. A vibration damped sandwich system as claimed in claim 1, wherein theratio of the thicknesses of the plates lies preferably in the range offrom 1:1 to 1:4.

References Cited UNITED STATES PATENTS 6 FOREIGN PATENTS 1,200,45811/1965 Germany.

OTHER REFERENCES Chemical Abstracts, volume 55:4036b.

MORRIS LIEBMAN, Primary Examiner S. L. FOX, Assistant Examiner U.S. Cl.X.R.

ll-165, 218; 260-80.8l, 885

